(a) Field of the Invention
The present invention relates to an alignment layer and a liquid crystal display including the same.
(b) Description of the Related Art
The liquid crystal display (“LCD”) is one of the most widely used flat panel displays. An LCD comprises two panels provided with field-generating electrodes such as pixel electrodes and a common electrode and a liquid crystal (“LC”) layer interposed therebetween. The LCD displays images by applying voltages to the field-generating electrodes to generate an electric field in the LC layer, which determines the orientation of LC molecules in the LC layer to adjust polarization of incident light.
In most liquid crystal displays the two field generating electrodes are respectively provided on each panel of the display. In such an arrangement, usually one of the two panels has a plurality of pixel electrodes arranged in matrix formation and the other panel has a common electrode covering almost the entire surface of the panel.
The LCD displays images by applying voltages to each pixel electrode. For this reason, thin film transistors (“TFTs”), which are triple terminal elements and are connected to the pixel electrodes to switch applied voltages to the pixel electrodes, are provided on the panel. Gate lines transmitting signals to control the TFTs and data lines delivering voltages to be applied to the pixel electrodes are also formed on one of the panels.
The TFTs allow or block transmission of data signals which are propagating along the data lines to the pixel electrode.
Additionally, each panel of the display has an alignment layer to orient the liquid crystal molecules of the LC layer.
The alignment layer is typically made of a polyimide material because of its excellent chemical resistance and LC alignment characteristics.
Polyimide has a low solubility. Therefore, polyamic acid, the precursor of polyimide in a liquid state, is coated to form an alignment layer and is processed to transform the coated polyamic acid into polyimide. However some of the coated polyamic acid remains after the processing step. Consequently, polyimide and polyamic acid coexist in the alignment layer.
However, polyimide is immiscible with polyamic acid, so they will not mix to form a heterogeneous structure within the alignment layer. Since the only force existing between polyimide and polyamic acid is the Van der Waals force, which is a weak attractive force between certain molecules, some portions of the polyimide alignment layer may come off by a mechanical force such as rubbing.
The fragments which come off from the alignment layer may attach to a rubbing cloth used to apply the alignment layer. This may cause further damage to the rest of the alignment layer. Additionally, the fragments of the alignment layer may drift free and become impurities in the LC layer. These impurities dishevel alignment of the liquid crystal molecules and cause a stained image on the resulting LCD.